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There is more to replacing windows than choosing the glass and the frame. Existing openings may need to be changed to accommodate new windows. Energy efficiency upgrades or stricter building codes may dictate which models you can choose from. Additional expenses required for carpentry may also determine whether you go with custom or standard window sizing. Either way, it’s best to review the options and compare the final costs before making the decision to go with custom windows.

Deciding to Go Custom When deciding to replace your windows, you must first determine whether you really need custom windows. If the openings are an irregular size, compare how much it will cost to make the hole bigger or smaller to accommodate a stock window. This is a better option if you’re putting in a smaller window, but since it will still require patching, filling, and trimming or the services of a contractor, this option may cancel out any savings from buying off the shelf. If you’re making the opening larger, you might have expensive adjustments to the load-bearing parts of the wall. In all, it’s best to have a professional review the situation and give you an estimate so that you can really compare numbers.

Non-standard sizing is not the only reason to go custom. Replacing existing windows that don’t complement the design of your house is easier with custom windows that allow you to buy whatever style or shape you want. Upgrading to energy-efficient models is another great reason to opt for custom windows, especially if you are replacing a treasured signature window like a bow, bay, or oriel. Custom ordering allows you to determine the features and measurements you need.

Before you place your order be sure to check out different manufacturers. A size, option, or shape that is non-standard at one company may be standard at another, says Jeffrey Lowinski, vice president for technology marketing at the Window & Door Manufacturers Association. A third company may have a standard size you can use with minor modifications. “Shopping around to find a standard size that fits your opening may save you some money,” Lowinski says. You may also want to see if using several standard windows, instead of a large custom one, works in your space.

When ordering custom windows, make sure you have them made with the energy-efficiency and performance options best suited for your climate and region. Custom windows are generally available in the same materials as standard windows—vinyl, wood, aluminum, fiberglass or composite—and with the same options, including coatings, gases, impact resistance and light-transmittance values. Also, ask about the warranty: Most companies provide the same warranty on their custom windows as on their standard ones. The custom windows should also maintain the same performance ratings assigned by industry groups as the standard products.

Replacement Windows or New Windows Once you’ve decided to go custom, you have several options. If the existing frame is in good condition, you may not have to replace the whole window; that can save you money and let you keep the same interior and exterior trims and appearance. You might be able to just order custom panes, unless they’re glued into the frame or you want to upgrade to a thicker glass. The next cheapest option is to order a custom sash—the removable glazing and support—if it’s compatible with the old frame. You can also order a custom replacement window, which is a new unit held together with a thin frame that’s inserted into the existing frame. The most expensive option is replacing the entire unit with a new window, including the frame, sash, and trims.

Cost There’s a good chance your custom windows will cost more and take longer to order than standard ones. “Generally you will pay a premium for a custom-sized product,” says John Lewis, technical director of the 400-member American Architectural Manufacturers Association. Still, it depends on the manufacturer and the flexibility of its equipment. Many companies do “just-in-time production,” which means they don’t make anything until they have a specific order. At these factories there may be little cost difference between standard and custom windows, and the price per unit will be the same whether they make one window or 50. However, at a company that has to set up the equipment to make a specific window, you might be able to negotiate a cheaper per-unit price by ordering a greater number. If that’s the case and you plan to replace more of the same windows in the future, it might be cheaper to order them all at once.

Measuring and Installing Unless you really know what you’re doing, it’s best to let an expert do the measuring before ordering your custom windows. “That’s a fatal error right there if you mis-measure the openings,” Lewis says. And once they’re made, you want to make sure they’re installed right. You can have the best, most expensive custom windows, but you will have wasted your money if they leak air or water, so make sure to have them professionally measured and installed.